in the Magnesian Limestone. 439 



" Concretionary Action," but it does not enable us any better to 

 realize the immediate cause of the process, or to understand the 

 manner in which it was brought about. With regard to the first part 

 of the problem, namely, the cause of the segregation, the following 

 considerations may throw a little light on the subject. 



The concretionary beds ai'e usually crowded with fossils, and it is 

 round these fossils that the carbonate of lime has segregated out ; 

 they, together with other foreign bodies (probably for the most part 

 organic particles, and now represented by cavities), were the, so 

 to speak, chemical magnets which attracted to themselves the 

 available carbonate of lime. 



The Roker beds, on the other hand, which are quite unfossiliferous 

 but are, as shown above, approximately identical in composition, 

 contain no concretions. 



It might be urged against this that the Middle Limestone, which 

 contains the most numerous assemblage of fossils in the whole 

 system, has developed no concretions ; but those beds contain a 

 relatively large proportion of carbonate of magnesia to carbonate 

 of lime, which, as we are about to point out, seems also to have 

 influenced the decision as to whether a certain bed should or should 

 not develope concretions. 



The presence of carbonate of magnesia in the unconsolidated 

 deposit appears up to a certain point to have assisted the segregation 

 of carbonate of lime ; but when the quantity of magnesia present 

 reached over 30 per cent., it has prevented this segregation from 

 taking place, except in small local patches. On the other hand, in 

 those beds which ai-e composed of almost pure carbonate of lime, 

 the concretionary structure is only developed in a minute form 

 throughout the mass. 



It would appear, then, from this that where the deposit was fairly 

 free from magnesia, the " concretionary action " started from so 

 many centres, that no centre had time to aggi'egate more than a 

 small quantity of lime to itself, before it came in contact with the 

 growing peripheries of neighbouring concretions, and its further 

 growth necessarily ceased. When, however, the deposit contained 

 more magnesia, the particles of lime were consequently more 

 widely separated, there was less inducement for individual effort 

 on the part of would-be centres, and only the larger and stronger 

 ones succeeded in attracting particles from the surrounding mass, 

 and grew in size in proportion to their numerical scarcity ; the 

 distance from which they were able to draw particles increasing 

 in proportion with the increase of their mass. When, however, the 

 proportion of magnesia present exceeded a certain amount, the 

 distance between the particles of Hme became so great, and their 

 mutual affinity was so weakened by this increased separation, that 

 no foreign body was sufficiently powerful to start the migration of 

 particles in its own direction, and the deposit solidified as an 

 ordinary compact bed. 



We have then in concretionary action a very analogous process to 

 crystallization. When the solution of a salt is concentrated and 



